The two-component negative charge developer for use in the electrophotography is usually a mixture of a negative charge toner and a positive charge carrier, wherein the carrier is used for the purpose of providing a proper amount of negative triboelectric charge to the toner.
As the carrier a resin-coated carrier consisting of core particles with their surface coated with a resin is suitably used.
In recent years, electrophotographic copying apparatus have a tendency toward being down-sized as seen in laser printers. With this trend, the developing device inside the image copying apparatus is sized small as well.
In the small-sized developing device, however, the quantity of a developer provided for developing the electrostatic latent image becomes necessarily reduced, so that, in the aforementioned two-component negative charge developer, the supplied toner is required to be given a proper amount of negative charge in a short period up to being transported to the developing region, i.e., required to have its charging rise time characteristic improved.
For this reason, the incorporation of a negative charge control agent into the toner has conventionally been prevailing as means to improve the charging rise time characteristic of the negative charge developer.
However, in the small-sized developing device holding a reduced amount of a developer, the addition of a charge control agent to the toner side alone is not enough for improving the charging rise time characteristic, thus resulting in an increase in the amount of weakly charged toner to cause toner scattering to produce fog on the resulting image.
As another means to improve the charging rise time characteristic in the negative charge developer there is introduced a technique to incorporate a positive charge control agent into the resin-coated phase of the resin-coated carrier as described in Japanese Patent Publication Open to Public Inspection (hereinafter abbreviated to JP O.P.I.) No. 8860/1990.
Those known as the positive charge control agent include the quaternary ammonium compounds disclosed in JP O.P.I. Nos. 51951/1974 and 10141/1977, and the alkylpyridinium compounds and alkylpicolium compounds, such as nigrosine SO, nigrosine EX.
These conventionally known positive charge control agents are largely cohesive organic compounds, so that they are inferior in the dispersibility/miscibility with resins for coating.
Therefore, the above positive charge control agent can not be uniformly incorporated into the carrier's resin coat phase, and thus the uneven distribution of the charge control agent or the extrication of the agent from the resin coat phase makes it unable to provide a proper amount of negative triboelectric charge to the toner, resulting in the toner scattering or fog trouble attributable to charging failure.
Further, in the case where the image formation is repeated many times by use of such the negative charge developer, the toner component material is liable to moltenly adhere to the carrier surface. And there also occurs another problem that the tonerspent causes the charge control agent present on the carrier surface to be covered with the toner component material to thereby temporarily lower the charging rise time characteristic.
Incidentally, there has lately been suitably used a positive charge photoreceptor capable of contributing to reducing the discharge amount of ozone and so good for protection of environment, particularly a positive charge type organic photoreceptor or amorphous silicone photoreceptor excellent also in the safety as its component material.
However, the photoreceptor has the disadvantage that its surface tends to be deteriorated due to the contact of it with ozone or paper dust that occurs at the time of the image forming operation. And the deteriorated surface of the photoreceptor invites the lowering of its cleanability (cleaning failure), and causes the resulting image to smear.
Accordingly, in order to prevent the photoreceptor from the cleaning failure and smear to retain its stable performance, it is necessary for the photoreceptor to have its surface polished to remove therefrom the deteriorated layer formed thereon to keep the surface always clean.
As a means to polish the photoreceptor's surface to remove the formed deteriorated layer therefrom, JP O.P.I. No. 81127/1978 discloses a technique for providing a polishing effect to the photoreceptor by the incorporation of an abrasive into the developer.
However, the technique of incorporating the abrasive into the developer is unable to exhibit sufficiently uniformly its polishing effect to photoreceptors having a high surface hardness, such as recent organic photoreceptors and amorphous silicon photoreceptors, and therefore it has not attained any certain measure for preventing the above cleaning failure and image smear problems. In this technique, there are cases where the abrasive extricated from the developer scratches the cleaning blade or stains the wire.